Method for the production of an initiator composition for retarded anionic polymerization

ABSTRACT

Disclosed are a process for the preparation of an initiator composition comprising an alkali metal organyl and an aluminum organyl and a process for the polymerization of anionically polymerizable monomers.

The present invention relates to a process for the preparation of aninitiator composition comprising an alkali metal organyl and an aluminumorganyl and to a process for the polymerization of anionicallypolymerizable monomers.

Anionic polymerizations typically proceed very rapidly, so that they aredifficult to control on an industrial scale owing to the considerableamount of heat generated. Lowering the polymerization temperatureresults in an excessive increase in viscosity, in particular with aconcentrated solution. Reducing the initiator concentration increasesthe molecular weight of the polymer formed. Controlling the reaction byappropriate dilution of the monomers results in higher solventrequirement and lower space-time yields.

It has therefore been proposed to include in the anionic polymerizationinitiators various additives to influence the polymerization rate.

The effect of Lewis acids and Lewis bases on the rate of the anionicpolymerization of styrene was described in Welch, Journal of theAmerican Chemical Society, Vol 82 (1960), pages 6000-6005. For instance,it has been found that small amounts of Lewis bases such as ethers andamines accelerate the n-butyllithium-initiated polymerization of styreneat 30° C. in benzene, whereas Lewis acids such as zinc and aluminumalkyls reduce the polymerization rate or, when used insuperstoichiometric amounts, stop the polymerization completely.

U.S. Pat. No. 3,716,495 discloses initiator compositions for thepolymerization of conjugated dienes and vinylaromatics where a moreefficient use of the lithium alkyl as initiator is achieved by theaddition of a metal alkyl of a metal of group 2a, 2b or 3a of thePeriodic Table of the Elements, such as diethyl zinc, and polarcompounds such as ethers or amines. The manner in which the individualinitiator components are added to the polymerization system is said tobe uncritical.

Earlier patent application PCT/EP97/04497, unpublished at the prioritydate of the present invention, describes continuous processes for theanionic polymerization or copolymerization of styrene or diene monomersusing alkali metal alkyl as polymerization initiator in the presence ofan at least bivalent element as a retarder.

Various initiator mixtures which may comprise alkali metals, alkalineearth metals, aluminum, zinc or rare earth metals are known, forexample, from EP-A 0 234 512 for the polymerization of conjugated dieneswith a high degree of 1,4-trans-linking. German Offenlegungsschrift 2628 380 teaches, for example, the use of alkaline earth aluminates ascocatalyst in conjunction with an organolithium initiator for thepreparation of the polymers or copolymers of conjugated dienes having ahigh trans-1,4-linkage content and low 1,2-linkage or 3,4-linkagecontents. This is said to lead to an increase in polymerization rate.

The use of additives such as aluminum alkyls which have a strongretarding effect on the anionic polymerization requires exact dosage andtemperature control. A slight underdosage may lead to an insufficientretardation of the reaction rate, whereas a slight overdosage maycompletely stop the polymerization.

Separate addition, or insufficient mixing-in, of the individualinitiator components to a monomer solution may lead to poor dispersion,particularly at high monomer concentrations, and house to differentlocal concentrations of the individual initiator components. Before ahomogeneous dispersion of the initiator components can be achieved, thepolymerization may already be initiated in some regions, whereas thepolymerization is strongly retarded or has not yet started in others.This may lead to large local temperature increases and irreproduciblemolecular weight distributions.

It is an object of the present invention to provide a process for thepreparation of an initiator composition comprising an alkali metalorganyl and an aluminum organyl to make it possible to polymerizeanionically polymerizable monomers, in particular styrene, in areproducible manner with respect to polymerization rate and molecularweight distribution.

We have found that this object is achieved by a process for thepreparation of an initiator composition comprising an alkali metalorganyl and an aluminum organyl, which comprises homogeneously mixingthe metal organyls, dissolved in inert hydrocarbons, and aging at atemperature in the range from 0 to 120° C. for at least 2 minutes.

The initiator composition prepared in this manner is particularly usefulfor the polymerization of anionically polymerizable monomers.

Alkali metal organyls which may be used are mono-, bi- ormultifunctional alkali metal alkyls, aryls or aralkyls customarily usedas anionic polymerization initiators. It is advantageous to useorganolithium compounds such as ethyllithium, propyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, tert-butyllithium,phenyllithium, diphenylhexyllithium, hexamethylenedilithium,butadienyllithium, isoprenyllithium, polystyryllithium or themultifunctional compounds 1,4-dilithiobutane, 1,4-dilithio-2-butene or1,4-dilithiobenzene. The amount of alkali metal organyl required dependson the desired molecular weight, the type and amount of the other metalorganyls used and the polymerization temperature and is typically in therange from 0.0001 to 5 mol percent, based on the total amount ofmonomers.

Aluminum organyls which may used are those of the formula R₃Al, whereinthe radicals R are each, independently of one another, hydrogen,halogen, C₁₋C₂₀-alkyl or C₆₋C₂₀-aryl. Preferred aluminum organyls arealuminum trialkyls such as triethylaluminum, triisobutylaluminum,tri-n-butylaluminum, triisopropylaluminum or tri-n-hexylaluminum.Particular preference is given to using triisobutylaluminum. It is alsopossible to use aluminum organyls which are formed by partial orcomplete hydrolysis, alcoholysis, aminolysis or oxidation of alkyl- orarylaluminum compounds or those which carry alkoxide, thiolate, amide,imide or phosphide groups. Examples are diethylaluminumN,N-dibutylamide, diethylaluminum ethoxide, diisobutylaluminum ethoxide,diisobutyl-(2,6-di-tert-butyl-4-methyl-phenoxy)aluminum (CAS No.56252-56-3), methylaluminoxane, isobutylated methylaluminoxane,isobutylaluminoxane, tetraisobutyldialuminoxane, orbis(diisobutyl)aluminum oxide.

The molar ratios of the metal organyls with respect to each other mayvary within wide limits, but depend primarily on the desired retardationeffect, the polymerization temperature, the monomer composition andconcentration and the desired molecular weight.

The molar ratio of aluminum to alkali metal is advantageously in therange from 0.2 to 4.

In the process of the invention, use is made primarily of alkali metalorganyls and aluminum organyls and, if desired, magnesium organyls.Barium, calcium or strontium organyls are preferably only present inineffective amounts not having a significant effect on thepolymerization rate or copolymerization parameters. Nor shouldtransition metals or lanthanoids, especially titanium, be present insignificant amounts.

The inert hydrocarbon used may be aliphatic, cycloaliphatic or aromatic.Preference is given to using solvents in which the metal alkyls arecommercially available in the form of a solution. Particular preferenceis given to using pentane, hexane, heptane, cyclohexane, ethylbenzene ortoluene.

The initiator components are advantageously used in the solutionconcentrations in which they are commercially available or, for aquicker establishment of equilibrium, in a more diluted form. Preferenceis given to concentration where the sum of all metal organyls is in therange from 0.01 to 2 mol/1, based on the initiator composition.

The temperature depends on the concentration, the type of the metalorganyls and the solvent. It is usually possible to use any temperaturebetween the freezing point and boiling point of the mixture. It isadvantageous to use a temperature in the range from 0 to 120° C.,preferably in the range from 20 to 80° C.

The aging of the metal organyls is important for their reproducible usein anionic polymerization. Experiments showed that initiator solutionswhich are mixed separately or just prior to the initiation of thepolymerization result in poorly reproducible polymerization conditionsand polymer properties. It is believed that the aging process observedis caused by a complexation of the metal organyls which proceeds slowerthan the mixing process. In the concentration and temperature rangesdescribed above, an aging time of about 2 minutes is usually sufficient.It is preferred to age the homogeneous mixture for at least 5 minutes,in particular at least 20 minutes. However, aging the homogeneousmixture for several hours, e.g. from 1 to 480 hours, is not usuallyharmful either.

It is also possible in the process of the invention to additionally addstyrene. In this case an oligomeric polystyryl anion is obtained withthe metal organyls complexed at its chain end. Preference is given tousing styrene in an amount in the range from 10 to 1000 mol%, based onthe alkali metal organyl.

The initiator components may be mixed in any mixing apparatus,preferably in those which may be pressurized with inert gas. Examples ofsuitable mixers are stirred tanks equipped with anchor stirrers orshaker containers. Heatable tubes equipped with static mixing elementsare particularly suitable for continuous preparation. The mixing processis necessary to mix the initiator components homogeneously. It ispossible but not strictly necessary to continue mixing during aging. Itis also possible to carry out the aging process in a continuous stirredtank reactor or in a tube section, the volume of which, together withthe flow rate, determines the aging time.

The initiator compositions are suitable for the polymerization ofanionically polymerizable monomers. The initiator composition ispreferably used for the homopolymerization or copolymerization ofvinylaromatic monomers and dienes. Preferred monomers are styrene,α-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene,vinyltoluene or 1,1-diphenylethylene, butadiene, isoprene,2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadiene or piperylene ormixtures thereof.

The amount of initiator composition required depends on the desiredmolecular weight, the type and amount of the further metal organyls usedin addition to the alkali metal organyl, and the polymerizationtemperature, and is usually in the range from 0.0001 to 5 mol%, based onthe alkali metal organyl content and the total amount of monomers.

The polymerization may be carried out in the presence of a solvent.Suitable solvents are the aliphatic, cycloaliphatic or aromatichydrocarbons having from 4 to 12 carbon atoms which are generally usedfor anionic polymerization, such as pentane, hexane, heptane,cyclohexane, methylcyclohexane, isooctane, decalin, benzene,alkylbenzenes such as toluene, xylene, ethylbenzene or cumene orsuitable mixtures. Obviously, the solvent must have the high puritywhich is typically required for the process. The solvents may be driedover aluminum oxide or molecular sieve and/or distilled prior to use toremove protic substances. The solvent from the process is preferablyreused after condensation and the abovementioned purification.

It is possible to adjust the retarding effects within wide temperatureranges via the composition and amount of the metal organyls. It istherefore also possible to carry out the polymerization at initialmonomer concentrations in the range from 50 to 100% by volume,particularly from 70 to 100% by volume, which lead to highly viscouspolymer solutions and require higher temperatures at least at higherconversions.

After the polymerization is completed, the living polymer ends may becapped with a chain terminator. Suitable chain terminators are proticsubstances or Lewis acids, such as water, alcohols, aliphatic oraromatic carboxylic acids and inorganic acids such as carbonic acid orboric acid.

The target products may be homopolymers or copolymers and mixturesthereof. Polystyrene and styrene/butadiene block copolymers arepreferably obtained. The process of the invention may also be used toprepare high-impact polystyrene (HIPS), in which case polybutadiene,styrene/butadiene block copolymers or mixtures thereof may be used asrubbers.

The block copolymers may be coupled using multifunctional compounds,such as polyfunctional aldehydes, ketones, esters, anhydrides orepoxides. The process of the invention may be carried out in anypressure- and temperature-resistant reactors, it being possible inprinciple to use backmixing or non-backmixing reactors (i.e. reactorshaving stirred tank or tubular reactor characteristics). Depending onthe choice of initiator concentration and composition, the particularprocess route applied and other parameters, such as temperature andpossible temperature profile, the process of the present invention leadsto polymers having high or low molecular weight. It is possible to use,for example, stirred tanks, tower reactors, tube reactors and tubularreactors or tube bundle reactors with or without internals. Internalsmay be static or mobile. The process is preferably carried outcontinuously.

The initiator compositions prepared according to the process of theinvention make it possible to control the polymerization of anionicallypolymerizable monomers, in particular styrene, very effectively and toachieve reproducible polymer properties.

EXAMPLES:

Preparation of Initiator Solution I1

8 ml of a 1.6-molar s-butyllithium solution (sBuLi) in cyclohexane (fromAldrich) and 6.4 ml of a 1.6-molar solution of triisobutylaluminum(TIBA) in toluene (from Witco) were combined at 25° C. and stirred priorto use for 10 hours.

Example 1

A 2.35 1 stirred tank equipped with an anchor stirrer was charged with800 g of styrene and 200 g of toluene under nitrogen and heated to 85°C. with stirring. On reaching this temperature, initiator solution I1(molar Li/Al ratio=1/0.85) was added and the polymerization solution waskept at 85° C. The conversion was 29% after 25 minutes. After 60 minutesat 85° C., the polymerization was terminated at a conversion of 52% byadding 4 ml of ethanol. The viscous polystyrene solution obtained had anumber average molecular weight M_(n) of 54,340 g/mol and apolydispersity M_(w)/M_(n) of 1.29.

Comparative Experiment 1

Example 1 was repeated, except that the components of initiator solutionI1 were combined and added to the monomer solution within less than 1minute. The polymerization mixture was heated up to 213° C. within 5minutes.

Example 2

1.5 ml of sec-butyllithium (1.3 M in cyclohexane) and 2.3 ml of styrenewere added to 30 ml of cyclohexane and the mixture was stirred for 4hours. Then 1.66 ml of a 1 M solution of triisobutylaluminum incyclohexane (molar Al/Li ratio=0.85) were added and the solution wasstirred at room temperature (25° C.) for a further 4 hours. Glassampoules were charged with 2.5 ml of this solution and 10 ml of styreneeach, fused and stored in a heating bath at 150° C. The ampoules wereopened at different times and the polymerization was terminated byadding ethanol. The time/conversion curve obtained was used to calculatea half-life of 1 minute for the styrene conversion at 150° C.

Example 3

Example 2 was repeated, except that 1.86 ml of a 1 M solution oftriisobutylaluminum in cyclohexane was used (molar Al/Li ratio=0.95).The half-life at 150° C. was more than 1 hour.

Comparative Experiment 2

0.75 ml of a sec-butyllithium solution (1.3 M in cyclohexane) and 0.6 mlof styrene were added to 200 ml of cyclohexane and stirred for 4 hours.30 ml of this solution were transferred to a 100 ml flask equipped witha fused-on UV cell. The concentration of the polystyryllithium asdetermined by UV spectroscopy was [PS-LI]=4.8×10⁻³ M. This solution wasmixed with 3.6 ml of a 0.06 M solution of Et₂AlOEt in cyclohexane (molarAl/Li ratio=1.5) and 2.5 ml of styrene. The decrease in the styreneconcentration at 100° C. was monitored by UV spectroscopy and analyzedaccording to a first order rate equation:

ln ([styrene]₀/[styrene])=k_(a) * t

A non-linear plot was obtained. The slope of the curve decreased withtime. Towards the end of the conversion, the slope k_(a) was 0.0035min⁻¹. This ka value and the polystyryllithium concentration [PS-Li]gave a reaction rate constant k_(p =k) _(a)/[PS-Li]^(0.5)>0.05 M^(−0.5)min⁻¹.

Example 4

6.2 ml of a 0.06 M solution of Et₂AlOEt in cyclohexane were added to 30ml of a solution of polystyryllithium in cyclohexane having a [PS-Li]concentration of 6.2 ×10⁻³ M as determined by UV spectroscopy andstirred at 100° C. for 1h (molar Al/Li ratio=1.5). Further 2.5 ml ofstyrene were then added. The decrease in styrene concentration at 100°C. was monitored by UV spectroscopy and analyzed as described inComparative Example 2. The plot of ln ([styrene]₀/[styrene]) as afunction of time was linear over the whole conversion range. The slopek_(a) of the straight line was 2.6=10⁻⁴min⁻¹. This k_(a) value and the(PS-Li] concentration gave a reaction rate constant k_(p) of 0.0033M^(−0.5)min^(−1.)

Example 5

1.2 ml of a sec-butyllithium solution (1.3 M in cyclohexane) and 0.9 mlof dry styrene were added to 200 ml of cyclohexane and stirred for 4hours.

30 ml of this solution were transferred to a 100 ml flask equipped witha fused-on UV cell. The concentration of the polystyryllithium asdetermined by UV spectroscopy was [PS-Li]=7.2×10⁻³ M. This solution wasmixed with 2.3 ml of a 0.08 M solution of triisobutylaluminum incyclohexane (molar Al/Li ratio=0.85). The UV-VIS spectrum of thesolution was monitored in the UV cell at room temperature.

An absorbence maximum of 287 nm was observed, which grew by 20% over aperiod of 2 h. After this period, a shoulder at about 330 nm had almostcompletely disappeared; the corresponding absorbence was reduced toabout 64% of the initial value over this period.

Example 6

A 1 l stirred tank equipped with an anchor stirrer was charged with 120g of styrene and 480 g of toluene under a nitrogen atmosphere and heatedto 80° C. with stirring. At the same time, an ampoule containing 10 mlof toluene and 0.5 ml of styrene was charged with 1.51 ml of a 1.6-molars-butyllithium solution in cyclohexane and, after 10 minutes, with 1.42ml of a 1.6-molar solution of triisobutylaluminum in toluene. Themixture was kept at 80° C. for 5 minutes and then added to the stirredtank. At a constant temperature of 80° C., the styrene conversion was14% after 60 minutes, 36% after 115 minutes and 63% after 181 minutes.After 360 minutes, the polymerization was terminated at a conversion of92% by adding 4 ml of ethanol.

Comparative Experiment 3

A 1 l stirred tank equipped with an anchor stirrer was charged with 120g of styrene and 480 g of toluene under a nitrogen atmosphere and heatedto 60° C. with stirring. On reaching this temperature, 1.51 ml of a1.6-molar s-butyllithium solution in cyclohexane and 1.42 ml of a1.6-molar solution of triisobutylaluminum in toluene were added at thesame time, but separately. After 3 minutes, the conversion was 51% andthe temperature had risen to 77° C. After 5 minutes, the conversion was61% and the temperature 72° C., and after 40 minutes, the conversion was83% and the temperature was 60° C.

Example 7

The reactor used for the continuous polymerization was a double-jacketed2 1 stirred tank equipped with a standard anchor stirrer. The reactorwas designed for a pressure of 60 bar and was kept at a specifiedtemperature by heat-transfer medium to allow an isothermalpolymerization. The initiator components were metered in via a commonfeed line using a static mixer. The feed line had a capacity of 160 ml,with a section containing 100 ml being kept at 80° C.

The stirred tank was continuously fed with 800 g/h of styrene and, viathe common feed line, with a premixed initiator solution comprising 26ml/h of a 0.16-molar s-butyllithium solution in cyclohexane/toluene(1/9), 24.7 ml/h of a 0.16-molar solution of triisobutylaluminum intoluene, 180 g/h of toluene and 24 g/h of a 10% strength by weightsolution of styrene in toluene (molar Li/Al ratio=1/0.92) and stirred(100 revolutions per minute) at a bulk temperature of 104° C. Theeffluent from the stirred tank was conveyed into a stirred 4 liter towerreactor which was operated at an internal temperature of 109° C. Theeffluent from the reactor was fed into a second 4 liter tower reactor.To set the temperature, two heating zones of equal length which werearranged in series were used, the internal temperature at the end of thefirst zone being 140° C., and at the end of the second zone being 158°C. The polymerization mixture was mixed with 20 g/h of a 10% strength byweight solution of methanol in toluene using a mixer at the outlet ofthe tower reactor, subsequently passed through a tube section heated to260° C. and released into a vacuum pot kept at 25mbar via a pressurecontrol valve. The melt was discharged via a screw conveyor andpelletized.

A stable equilibrium state was reached in all parts of the unit after afew hours. The pressure drop across the whole unit was 2.2 bar. Thesolids content was 13.5% by weight at the outlet of the stirred tank and40.4% by weight at the outlet of the tower reactor. The monomerconversion of the effluent was found to be complete. The polystyreneobtained had a molecular weight Mw of 167,000 g/mol and a polydispersityM_(w)/M_(n) of 2.62. The distribution was monomodal. Analysis showed astyrene content of less than 10 ppm, an ethylbenzene content of lessthan 10 ppm and a toluene content of 92 ppm.

Comparative Experiment 4

Example 7 was repeated, except that 26 ml/h of a 0.16-molars-butyllithium solution in cyclohexane/toluene (1/9), 24.7 ml/h of a0.16-molar solution of triisobutylaluminum in toluene and 180g/h oftoluene (molar Li/Al ratio=0.95) were metered in via separate feedlines. When attempting to adjust the solids content to 13.5% by weightat the outlet of the stirred tank, the temperature had to be decreasedto 83° C. The solids content in the stirred tank varied in the rangefrom 3-25% by weight in the course of several days. The bulk temperaturecould not be kept constant at 83° C. Samples taken from the stirred tankshowed significant variations in molecular weight distribution andsometimes bimodal or multimodal distributions.

Example 8

The initiator components were fed to a thermostatable coil having alength of 4m via a mixing element. The coil had a volume of 12.6 ml andled to the reactor.

The stirred tank from Example 7 was continuously fed with 800 g/h ofstyrene, 180 g/h of toluene and, via the common feed line, with apremixed initiator solution comprising 23 ml/h of a 0.18-molars-butyllithium solution in cyclohexane/toluene (1/9) and 44.3 ml/h of a0.086-molar solution of triisobutylaluminum in toluene (molar Li/Alratio=1/0.92) and stirred (100 revolutions per minute) at a bulktemperature of 109° C. A constant operational state was reached afteronly a few hours. The solids content was 14%.

Comparative Experiment 5

Example 8 was repeated, except that the metal alkyls were introducedinto the reactor via a feed line with a capacity of only 0.9 ml. Thesolids content in the reactor rose to 41% within only a few hours. Itwas found to be difficult to keep the solids content in the stirred tankand the bulk temperature at a constant level.

We claim:
 1. A process for the preparation of an initiator compositioncomprising an alkali metal organyl and an aluminum organyl excludingbarium, calcium or strontium, which comprises homogeneously mixing themetal organyls, dissolved in inert hydrocarbons, and aging at atemperature in the range from 0 to 120° C. for at least 2 minutes.
 2. Aprocess as claimed in claim 1, wherein the alkali metal organyl used isa lithium organyl.
 3. A process as claimed in claim 1, wherein the inerthydrocarbons used are pentane, hexane, heptane, cyclohexane,ethylbenzene or toluene.
 4. A process as claimed in claim 1, wherein theconcentration of the sum of all metal organyls is in the range from 0.01to 2 mol/l, based on the initiator composition.
 5. A process as claimedin claim 1, wherein the molar ratio of aluminum to alkali metal is inthe range from 0.2 to
 4. 6. A process as claimed in claim 1, whereinadditionally styrene is added before aging to form an oligomericpolystyryl anion in an amount in the range from 10 to 1000 mol %, basedon the alkali metal organyl.
 7. A process for the polymerization ofanionically polymerizable monomers, which comprises preparing aninitiator composition by the process of claim 1 and polymerizinganionically polymerizable monomers in the presence of the initiatorcomposition.
 8. A process as claimed in claim 7, wherein the anionicallypolymerizable monomers are styrene.